Both synthetic and natural zeolites and their use in promoting certain reactions, including the selective reduction of nitrogen oxides with ammonia in the presence of oxygen, are well known in the art. Zeolites are aluminosilicate crystalline materials having rather uniform pore sizes which, depending upon the type of zeolite and the type and amount of cations included in the zeolite lattice, range from about 3 to 10 Angstroms in diameter. Levyne (LEV) is a small pore zeolite with 8 member-ring pore openings (˜4.8×3.6 Angstroms) accessible through its 2-dimensional porosity (as defined by the International Zeolite Association). A cage like structure results from the connection of double six-ring building units by 4 rings.
Levyne can be synthesized using various template agents and OH-sources. These various synthesis routes result in Levyne-type materials with different names such as Levyne, LZ-132, LZ-133, Nu-3, ZSM-45, ZK20, SSZ-17. U.S. Pat. No. 3,459,676 first disclosed the synthesis of ZK-20 having a silica to alumina ratio from 4 to 11 using 1-methyl-1-azonia-4-azabicyclo[2,2,2]octane. EP 91,048 and EP 91,049 describe the synthesis of LZ-132 and LZ-133 using methylquinuclidine. EP 40,016 describes the synthesis of Nu-3 (10 to 300 SiO2:Al2O3) with 1-aminoadamantane or methylquiniclidine. EP 107,370, U.S. Pat. No. 4,485,303, U.S. Pat. No. 4,086,186 U.S. Pat. No. 5,334,367, describes the synthesis of ZSM-45 (10 to 80 SiO2:Al2O3) with salts of dimethyldiethylammonium, choline or cobaltinium. Caullett et al. described the synthesis of Levyne with quinuclidine and methylamine in Zeolites, 1995, 15, 139-147. Touto et al., describe the synthesis of Levyne with methylquinucline in Materials Engineering, 1994, 175-182 and Microporous and Mesoporous Materials, 1998, 247-257. Inoue et al. describe the hydrothermal conversion of FAU to Levyne with choline hydroxide in Microporous and Mesoporous Materials, 2009, 149-154.
The reduction of nitrogen oxides with ammonia to form nitrogen and H2O can be catalyzed by metal-promoted zeolites to take place preferentially to the oxidation of ammonia by the oxygen or to the formation of undesirable side products such as N2O, hence the process is often referred to as the “selective” catalytic reduction (“SCR”) of nitrogen oxides, and is sometimes referred to herein simply as the “SCR” process.
The catalysts employed in the SCR process ideally should be able to retain good catalytic activity over the wide range of temperature conditions of use, for example, 200° C. to 600° C. or higher, under hydrothermal conditions and in the presence of sulfur compounds. High temperature and hydrothermal conditions are often encountered in practice, such as during the regeneration of the catalyzed soot filter, a component necessary for the removal of soot particles in the exhaust gas treatment system.
Metal-promoted zeolite catalysts including, among others, iron-promoted and copper-promoted zeolite catalysts, for the selective catalytic reduction of nitrogen oxides with ammonia are known. Iron-promoted zeolite beta (U.S. Pat. No. 4,961,917) has been an effective commercial catalyst for the selective reduction of nitrogen oxides with ammonia. Unfortunately, it has been found that under harsh hydrothermal conditions, for example exhibited during the regeneration of a catalyzed soot filter with temperatures locally exceeding 700° C., the activity of many metal-promoted zeolites begins to decline. This decline is often attributed to dealumination of the zeolite and the consequent loss of metal-containing active centers within the zeolite.
WO 2008/106519 discloses a catalyst comprising: a zeolite having the CHA crystal structure and a mole ratio of silica to alumina greater than 15 and an atomic ratio of copper to aluminum exceeding 0.25. The catalyst is prepared via copper exchanging NH4+-form CHA with copper sulfate or copper acetate. The catalyst resulting from copper sulfate ion-exchange exhibits NOx conversion from 45 to 59% at 200° C. and ˜82% at 450° C. Copper acetate exchange results in a material with NOx conversion after aging of 70 and 88% at 200 and 450° C., respectively. These materials offer improvement in low temperature performance and hydrothermal stability in comparison to FeBeta. However, Chabazite remains an expensive material due to the cost of the trimethyladamantyl ammonium hydroxide necessary for its synthesis.
WO 2008/132452 discloses a number of zeolite materials that can be loaded with iron and/or copper with improvements in NOx conversion compared to Fe/Beta, Cu/Beta and Cu/ZSM-5. Example 2 indicates Cu/Nu-3 (a Levyne-type material) as such a material. This example states that an ammonium exchange was carried out before an aqueous copper exchange using copper nitrate. It is stated that multiple aqueous ion-exchanges were carried out to target 3 wt % Cu (3.76 wt % CuO). No details of the ion-exchange experiments are disclosed. Additionally, no details of critical composition parameters for the zeolite are given such as SiO2:Al2O3 or alkali metal content. As indicated above Nu-3 can be synthesized with a wide range of SiO2:Al2O3 (10 to 300). Example 6 indicates that the material is aged at 750° C. in 5% steam for 24 hours. FIG. 5 and FIG. 6 indicate the SCR performance of Cu/Nu-3 fresh and aged with comparison to other materials such as Cu/SAPO-34. FIG. 6 indicates that following hydrothermal aging the NOx conversion at 200 and 450° C. are significantly inferior to the Chabazite-type SAPO-34 technology after aging, with ˜60% versus ˜75% NOx conversion at 200° C. and ˜60% versus ˜80% at 450° C. However, no clear mention of test conditions for Cu/Nu-3 can be found.
Briend at al. report that SAPO-34 was unstable to a humid environment at temperatures below about 100° C. as reflected in a loss of structure (J. Phys. Chem., 1995, Vol. 99, p 8270-8276). However, at temperatures above 100° C. stability was not an issue.
Poshusta et al. observe an instability to humidity at low temperature with SAPO-34 membranes (J. Membrane Science, 2001, Vol. 186, p 25-40).
WO 2008/118434 indicates that a Levyne material that can retain at least 80% of its surface area and micropore volume after hydrothermal aging at 900° C. in 10% steam for 1 to 16 hours would be suitable for application in SCR. However, no synthesis or catalytic data are disclosed.
WO 2010/043891 indicates small pore zeolites (having a maximum ring size of eight tetrahedral atoms), including Levyne (LEV), as improved catalysts in the selective catalytic reduction of NOx with ammonia. Levynite, Nu-3, LZ-132 and ZK-20 are reported. It is indicated that large crystal size results in improved catalyst stability with catalytic data provided for only Cu/Chabazite. NOx conversion is reported at 200° C. and 400° C. Crystals larger than 0.5 micrometers are claimed.
U.S. Pat. No. 4,220,632 discloses NH3—SCR process using zeolites in the Na- or H-form with pore sizes of 3-10 Angstroms. Zeolite X, Mordenite and a natural zeolite are disclosed in the examples.